The invention relates to an alloy system applied as a coating on metal tubes used in high temperature applications such as ethylene production to resist carburization, inhibit catalytic coke formation, and resist coke fouling.
Ethylene is produced by passing a feedstock containing naphtha, ethane, and other distillates through a furnace comprised of a series of tubes. In the production of ethylene there are components that operate at elevated temperatures such as the cracking furnace, transfer piping, quench exchangers and transfer line exchangers (TLEs). These components are exposed to a high temperature environment that can operate in reducing or oxidizing or alternately between both conditions. To achieve desired creep strength, mechanical requirements, and oxidation resistance, the furnace tubes are made of higher alloys such as the wrought Alloy 800 series, and centrifugally or static cast alloys such as HK, HP and 45Nixe2x80x9435Cr alloys. The feedstock enters the furnace at a temperature of about 1000xc2x0 F. (540xc2x0 C.) where it is heated to about 1650xc2x0 F. (900xc2x0 C.). During the process pyrolytic coke is produced. Some of the coke accumulates on the walls of the furnace tubes. Nickel and iron in the tubes react with the coke to form long whisker-like structures that extend from the walls of the tubes called catalytic coke. These strands tend to catch pyrolytic coke passing through the tubes to form a complex amorphous coke layer on the inner wall of the furnace tubes. This amorphous coke layer acts as an insulator increasing the temperature of the inner walls in order to deliver adequate heat to the process stream to crack the feedstock. Consequently, the furnace must be periodically cleaned to remove this layer of coke. This cleaning is often called de-coking. At many locations the tubes must be cleaned every 30 days.
1. Brief Description of the Prior Art
The art has attempted to control catalytic coking by the selection of high chromium and nickel alloys with significant silicon content or by applying a chromium or aluminum or ceramic coating to the inner walls of the furnace tube. However, higher chromium coatings introduce instability in the alloy structures. Aluminum coatings have found limited success on wrought alloys with process temperatures not exceeding 1650xc2x0 F. (900xc2x0 C.). At higher temperatures inter-diffusion and spalling can occur. Solid ceramic coatings suffer from cracking and spalling.
Coatings of two or more materials have also been proposed for metals used in high temperature process applications. Bessen in U.S. Pat. No. 4,087,589 discloses methods for applying a chromium coating, an aluminum coating or a chromium coating followed by an aluminum coating on a nickel base alloy. In Japanese Patent 80029151 there is disclosed a method for applying a chromium-aluminum-silicon coating. This coating is produced by a chromium pack cementation process followed by an aluminum-silicon pack cementation process. The coated metal is said to be useful for jet engines, gas turbines and internal combustion engines. In U.S. Pat. No. 3,365,327 there is disclosed a method for vapor diffusion coating metallic articles with aluminum-chromium-silicon to provide elevated temperature corrosion resistance for gas turbine and oil refinery applications. The technique involves a slurry coating followed by high temperature firing. There is no teaching in any of these references that such coatings would be useful for ethylene furnace tubes.
In our U.S. Pat. No. 5,873,951 we disclose a method for coating ethylene furnace tubes made from iron-nickel-chromium alloys in which we apply a chromium diffusion coating, clean and roughen that coating, apply a second coating containing aluminum that also has a nickel and iron-rich overlay and then we polish the second coating to remove that overlay. In our U.S. patent application Ser. No. 09/255,596 now U.S. Pat. No. 6,139,647 we disclose a method of coating products formed from an iron-nickel-chromium alloy in which we expose the surface of the alloy to hydrogen to remove diffusion limiting oxides and then diffuse chromium or other metals onto the prepared surface.
The prior art as a whole, thus, teaches various methods of applying coatings containing chromium or aluminum to nickel based alloy tubes and other products. The references also report the thickness of the coatings and even the principal elements in the coatings. Many of the prior art chromium and aluminum coatings have been effective to some extent in resisting corrosion and in reducing carburization and coking problems. Nevertheless, these coatings continually fail after some period of time, and others must be cleaned at regular intervals that could be as short as 30 days. Thus, there is a need for coatings which last longer and resist fouling or coke build-up for longer periods of time.
The failure of the art to develop longer lasting coating systems, particularly ethylene furnace tubes is the result of a lack of understanding of what happens to these coatings over time. Although several people have attempted to understand and explain why chromium or aluminum based coatings fail in high temperature applications, there has been only limited understanding of what makes a long-lasting coating for ethylene furnace tubes and other products exposed to a high temperature environment in reducing or oxidizing environments. We have learned that a greater understanding of these systems could only come from a long term study of chromium and aluminum based coatings on tubes used in an ethylene production furnace.
2. Carburization and Catalytic Coking
The production process of making ethylene from hydrocarbon feedstocks, such as ethane, propane, naphtha and mixed precursors, creates violent thermal cracking of the hydrocarbon feedstock causing the liberation of carbon species. The carbon is in the form of CO (carbon monoxide) and CO2 (carbon dioxide) which has the propensity to diffuse into the base material. This action of carbon ingress causes the depletion of chromium due to the formation of chromium carbides and it exposes the other elements in the base material to the process environment.
The stages and mechanisms of carburization in iron-nickel-chromium alloys are taught in the paper xe2x80x9cCarburization of high chromium alloysxe2x80x9d by Ramanarayanan, Petkovic, Mumford and Ozekein, as well as in the paper xe2x80x9cCarburizationxe2x80x94introductory surveyxe2x80x9d by Rahmel, Grabke and Steinkusch. Ramanarayanan et al. discuss the oxygen chemical potential and the carbon chemical potential, the key point being that there must be enough of an oxide (Cr2O4-spinel or Cr2O3) to resist the ingress of carbon. Rahmel et al. discuss the need for sufficient chromium on the surface, and the instability of chromium oxides with materials that have less than 30 weight percent chromium in their base metallurgy. The solubility of carbon in chromium is determined by the amount of chromium in the substrate alloy. For example, the solubility of carbon in an alloy containing 32 weight percent chromium is less than 0.02 weight percent carbon. The solubility of carbon in chromium approaches zero when the chromium content is approximately 40 weight percent. Based on these data, the conclusion that can be drawn here is that if there is a stable Cr2O3 layer then there will be no carbon ingress or permeation into the base material.
The exposure of nickel and iron to such high carbon activity process gases is known to cause what is called catalytic coking, growing from the metal species to form filamentous coke. This filamentous coke is most detrimental to the process efficiencies because there is extra CO generated and there is a rapid collection of naturally occurring thermal coke adhering and building up on the tube wall. This coke build-up creates short run times and limits the amount of ethylene that can be produced with a given amount of energy input. There is also the problem of long decoke times of a furnace due to the presence of catalytic coke and excess carbon in the base metal. All of these problems increase over time due to the continued diffusion of carbon into the base metal.
3. Our Long-term Study
We have done metallographic evaluations of ethylene furnace tubes coated in accordance with the methods disclosed in our above-identified U.S. Pat. No. 5,873,951 and application Ser. No. 09/255,596 now U.S. Pat. No. 6,139,647 after 27 months in-service. Through performance of several long term aging tests, we now understand how the diffusion coating system functions to stop carburization, to reduce catalytic coke formation, and to smooth the surface, acting as an anti-stick interface for the process and available carbon. The aluminum oxides that are formed as part of the oxidizing decoke process that is performed on the furnace tubes in effect burns off the carbon by oxidation, all the while oxidizing the aluminum rich outer layer. But that is not what transpires over the long term. According to Fick""s law, chromium naturally migrates within an alloy when the concentration gradient is sufficient and the temperatures promote high diffusivity. This usually occurs around 750xc2x0 C. (1380xc2x0 F.) and higher temperatures. Prior art chromium coatings have contained in excess of 10 to 20 weight percent chromium in the initial diffusion layer to change this zone from austenite to a very high temperature stable phase. The art also follows this first step with a predominantly aluminum diffusion second step that uses the chromium step as its barrier to continual diffusion. While this is all true and is proven in short term aging, we have discovered that the effectiveness of the diffusion system is defined by the aluminum combining with nickel and iron that have diffused counter to both the chromium and aluminum. These nickel and iron aluminides form a barrier to further chromium diffusion to the surface. The art has known about the ability of the initial chromium to limit continued aluminum diffusion, however it did not know how the aluminum second step limited the chromium diffusion.
Our long term aging study has also identified another attribute of chromium and aluminum diffusion coatings. That is the ability of the diffusion system to allow small amounts of chromium to migrate through the metal aluminides and form a most stable chromium layer on the very surface. This chromium layer has been known to contain from 60 to 90 weight percent chromium and range from less than 0.00004 inch (1 micrometer) to 0.00024 inch (6 micrometers) in thickness, which characterizes the art. Godlewski and Godlewska teach in the paper xe2x80x9cEffect of Chromium on the Protective Properties of Aluminide Coatingsxe2x80x9d that chromium present in the surface layers of aluminide coatings has a beneficial effect on their resistance to oxidation and hot corrosion. Additionally, careful studies such as taught in the paper xe2x80x9cCarburization of High Chromium Alloysxe2x80x9d by Ramanarayanan et al. have identified that the art develops a supporting special crystalline spinel structure just below the very surface of the new chromium layer. This crystalline spinel structure which is also formed over time at elevated temperatures further provides support to the new chromium layer. The art of the conversion of the diffusion coating system included the available spinel forming metals just below the chromium and the actual formation of a thin spinel layer via a supporting crystalline compound. We have observed that it is the manganese, iron, aluminum and silicon, which are available just below the new chromium layer, that enables the chromium layer to tolerate the elevated temperatures seen in ethylene steam crackers.
Prior art has taught that diffusion coatings are stable when applied to iron-, nickel-, and cobalt-base alloys containing alloying elements such as chromium, manganese, titanium, nitrogen, niobium, tungsten, aluminum, and silicon. Yet, the art failed to recognize that chromium, silicon and aluminum must all be present in the coating in sufficient amounts to create an effective long lasting coating for tubes used in ethylene furnaces. As an example, Godlewska and Godlewski have taught in the paper xe2x80x9cChromaluminizing of Nickel and Its Alloysxe2x80x9d that a two step chromium-aluminum diffusion coating can be formed on nickel alloys. However, spallation occurs in the coating because the aluminum content of their coating system is too high. Petrone, Mandyam and Wysiekierski teach in U.S. Pat. No. 6,093,260 a two layer chromium-silicon-titanium-aluminum coating system which may or may not contain aluminum in the outer layer, and never contains more than 15 weight percent aluminum. However, their aluminum concentration is too low to provide the exceptional resistance to catalytic coke formation afforded by an aluminum diffusion coating as taught in the present invention. Rapp, Wang, and Pangestuti teach in U.S. Pat. Nos. 5,492,727 and 5,589,220 a method for the simultaneous deposition of chromium and silicon. However, the key element aluminum is not present in this diffusion coating system. Woerde, Zimmermann, Steurbaut, Van Buren, and Gommans teach in PCT WO/97/16507 a process for diffusion of chromium followed by diffusion of aluminum. A similar process is disclosed by Rairden, III in U.S. Pat. No. 3,874,901. However, the phase stabilizing and highly carburization resistant element silicon is not included in either diffusion process. Silicon plays two major roles in the improved diffusion coating system taught here. First, it considerably slows down the formation of a blocking chromium carbide layer during the chemical vapor deposition (CVD) or physical vapor deposition (PVD) process, thus allowing enhanced deposition of pure chromium into the substrate alloy. Second, it allows the chromium to diffuse more deeply into the substrate alloy during the high temperature diffusion process.
It is also taught in the literature that these diffusion coating systems are steady state. We have discovered that there can be a metastable as-diffused coating system, which is altered or aged at elevated temperature to a more stable system. This is accomplished by transforming the coating system via the preferential migration of chromium and manganese through diffusion barriers to establish the final condition of the diffusion coating system that can stop carburization and catalytic coke formation. Based upon our analysis of furnace tubes and fitting samples that were removed from an ethylene steam cracker in-service that had experienced uncharacteristically high metal temperatures we have gained greater understanding of the coatings. From that understanding we are now able to disclose and define effective coatings that can operate in high temperature environments while being exposed to oxidizing conditions or reducing conditions. More particularly, by observing how these coatings change over time, we are now able to tell the art what an effective coating can contain. Then, prior art coating processes can be modified and controlled to create such coatings.
We have discovered that the key to the successful performance of the patented diffusion coating system, as described in U.S. Pat. No. 5,873,951, is not just in the steps identified in the diffusion system, nor just in the synergistic combination of elements. Rather, there is a unique conversion of the diffusion system. This conversion is accomplished by heating the diffusion system between either an aging process or in situ operation in an actual ethylene steam cracker furnace. The present diffusion coating system calls for the first step of chromium and silicon diffusion followed by a second step aluminum or aluminum and silicon diffusion. This in itself does not totally protect the base material from carburization and catalytic coke formation. Heating the diffusion coating system for a period of time in the 1292xc2x0 F. to 2102xc2x0 F. (700xc2x0 C. to 1150xc2x0 C.) temperature range allows for a balanced amount of chromium to migrate through the second step layer of aluminum to form a high concentration of chromium on the surface. This chromium is supported by spinel forming metal elements of iron and manganese from the base metal and aluminum and silicon in the coating. The new surface formed by this migration is stable, noted by the lack of depletion of chromium from the first layer and the lack of depletion of aluminum from the second layer.
We provide an improved diffusion coating system of a type which stops carburization of the base metal and which stops the formation of catalytic coke. We also provide a surface roughness that is greatly reduced from the as machined, as cast or as produced tube and fitting. Surface roughness measurements have been reduced from 125 AAH to 300 AAH (average arithmetic height) in the as produced condition to 20 AAH to 40 AAH after aging or time in service. The diffusion coating system can be applied to furnace tubes, fittings, outlet piping, transfer line exchangers, quench headers and any component that is exposed to a high temperature hydrocarbon. This diffusion coating system can be applied by either chemical vapor deposition (CVD) or physical vapor deposition (PVD), or by a combination of both. Whatever method is used, we deposit a sufficient amount of chromium, chromium-silicon or multiple combinations thereof followed by aluminum, aluminum-silicon or multiple combinations thereof. The surface of the part, usually a tube, is first prepared by cleaning to remove all rust, corrosion products, scale and loose debris in preparation for exposing the surface to diffusion or deposition of desired elements. The chromium, aluminum, and silicon, and possibly other coating materials such as manganese, are CVD or PVD deposited in a single step or alternatively by multiple metal layers. The metal layers are either diffusion heat treated in situ or are layered with subsequent intermediate PVD deposited chemistry such as silicon. The single metal layer or multiple metal layers are diffusion heat treated into the base material of iron-nickel-chromium alloys such as stainless steels. The alloys may be either wrought or cast. The diffusion thickness varies from 0.002 inch (0.0508 millimeter) to 0.030 inch (0.762 millimeter) with resultant maximum targeted from 0.012 inch (0.3048 millimeter) to 0.015 inch (0.381 millimeter). The CVD transport methods can be pack cementation, slurry or ceramic carrier, while the PVD transport method can utilize solid master alloys, sintered powder, powder, inserts, slurry, or ceramic cartridge. The PVD method results in a slightly smoother surface than the CVD method. Weld overlay processes and thermal spray processes may also be used to deposit the single or multiple layers. Post process polishing is an option but not entirely necessary.
Other objects and advantages of the present invention can be understood from a description of certain present preferred embodiments shown in the drawings.